Photovoltaic or “solar panels” are an example of a non-linear power source. Typically in solar panels, bypass diodes, controlled FETs and other bypass schemes have been used in the prior art to limit the power dissipated by a partially shaded photovoltaic (“PV”) solar cell or cells. The worst-case condition is an individual partially-shaded cell blocked whilst all other cells in the PV are fully illuminated. Looking to FIG. 1, representative of a typical PV 100, including bypass diodes 102.1 to 102.3 (referred to hereinafter collectively as “102”), a single solar cell, for example the cell denominated as element 106, may become shaded and therefore reverse biased while still conducting appreciable photo current, driven by the other cells in the same string. This may lead to power dissipation in the shaded cell 106 of as much as twenty times normal forward power dissipation. Due to the possibility of such excess power dissipation and an associated temperature increase of the cell and surrounding glass, the buckskin, laminate, solder joints and other components are made very rugged, leading to increased cost, along with a reduction of reliability. Similarly, small defects common in PV solar cells can conduct large amounts of localized current, causing very high power densities. For example, a single avalanche defect involving a site of just 156 microns on a side within a 156 mm on a side single PV solar cell would increase power density at the defect site to one million times greater than the typical reverse voltage with photo current. Avalanche, dislocation, micro cracks and other cell defects which may be considerably smaller than 156 microns on a side have been known to result in holes burning through a cell and solar panel encapsulating layers and in dripping very hot silicon, metal and organic compounds onto rooftops, plants and other combustible materials. The catastrophic nature of this risk leads solar panel manufacturers to incur considerable test costs and yield losses. Despite the PV industry's best efforts, some of these defects still occur in the field. For example, increased cyclic mechanical stress caused by large daily power dissipation swings of partially shaded cells may lead to micro cracking and/or a reduced avalanche breakdown voltage even though no such defect existed when the PV was manufactured.
In the prior art, methods and apparatus for preventing hot spots from occurring have been suggested. For example Kernahan, in U.S. Pat. No. 8,050,804, discloses a method wherein characterization data is taken from a given solar panel by a flash tester. The characterization data is used during operation and adjusted for temperature to control the solar panel current with the expectation that no solar cell is caused to become reverse biased. However such approaches may fail in that the method, or electronic module by which the method is performed, has no means for determining any change in characteristics after a solar panel has been tested. For example, a given solar cell may later develop microcracks, severely lowering its breakdown voltage and/or making part of the cell inactive, undetected by the disclosed control module. As a result, the module may allow or even cause a cell to become reverse biased, thereby creating a hot spot.
The need for high avalanche voltage leads panel manufacturers to use high quality (known high avalanche break down voltage) cells, keeping material costs high by forbidding the use of cells with lower break down voltages or cells made from less pure materials. What is needed is a way to construct a solar panel without bypass diodes wherein no solar cell, regardless of instant condition, may become reverse biased.
FIG. 2 is an example of an idealized voltage-current (VI) and voltage-power graph for a photovoltaic panel in an arbitrarily defined scenario wherein most cells have 1.0 sun insolation (the current along curve 224) and one or more cells have a 0.6 sun insolation due to shading (the current along curve 226) for one string of a PV configured according to FIG. 1. Being in series electrically, all cells in the string will have identical current, so each cell will develop a voltage according to its individual insolation level. Thus we see a voltage V0.6 (232) and V1.0 (234) corresponding to the common current 222. The graph of FIG. 2 is deemed idealized because it would be a simple matter for an inverter to hold the PV 100 at its maximum power point in that the power delivered, defined as V*I, is constant at a given temperature, as indicated by P1.0 (228) and P0.6 (230). As temperature increases, the power and current curves would move down.
FIG. 3 is an example of current and power characteristics of a typical PV 100. The family of power curves 324, 326 correspond to the VI curves for 1.0 sun 324 and 0.6 sun 326 insolation. A power solution corresponding to current I1 (330) is not possible, in that the cell with 0.6 sun insolation cannot provide enough current 342. The solution I2 (322) is be possible in that the current for 0.6 sun 342 and for 1.0 sun 340 can a common current value of 12 (322). However at current I2 (322) the series-connected cells providing voltage V1.0 in series with a single cell at V0.6 may cause the cell receiving less radiance to be reverse biased. In that condition the other cells (at V1.0) may have enough power to damage, even causing catastrophic failure of, the cell at V0.6; that is, the cell may be over powered by the other cells such that it cannot provide the voltage V0.6. Therefore a problem with the control of conventional solar panels is the possibility of a spread between the insolation, hence voltage, levels of fully exposed cells and one or more cells that are shaded.
Inverters and other panel controllers are used for operating a panel at its maximum power point condition. Many of these use a method denominated “perturb and observe” (PAO), wherein the controller modifies the load condition, for example to draw more current, then determines the power output of the panel. If the power has increased, the controller keeps changing the conditions in the same direction until the power diminishes, at which point it returns to a previous condition, deemed to be a new maximum power point condition. If the first experiment caused the power to go down, then the experiment is repeated in the opposite direction.
In a partial shading situation, it is possible for the perturb and observe method to create a condition beyond which the bypass diodes are effective, thereby damaging the solar panel. Even without damage, panels constructed using bypass diodes sacrifice some amount of electrical power generation efficiency, as well as areal efficiency, simply by the presence of the bypass diodes.
Solar panel 100 of FIG. 1 and solar panel 400 of FIG. 4 may be installed in any orientation. For simplicity and clarity of discussion the orientations shown will be referred to as the “up” or “upper portion” at the top of the page with the text is viewed right side up.
Partial shading of a PV 100 may result from a variety of causes, such as bird droppings and leaves to name a few. PVs 100 are obviously exposed to the elements, including dust and condensation. Condensation and rain will cause dirt accumulated on a panel to flow to the lower portion of the panel, obscuring the lower cells 108.